So-called compression ignition engines employ compression ratios that are much higher than Otto cycle (spark-ignited) engines. Characteristics of compression ignition engines such as direct injection, higher compression ratios and unthrottled air intake systems permit more efficient fuel combustion, higher performance, and lower fuel consumption on an energy basis, compared to spark-ignited engines. The most common compression ignition engines are diesel engines. However, in many countries, to comply in the future with already announced government regulatory standards, diesel engine manufacturers will be required to make improvements to engines being sold today in order to lower engine emissions of combustion products such as NOx and particulate matter. Such improvements are expected to require additional equipment and more sophisticated electronic engine controls to implement technology such as exhaust gas recirculation, new combustion strategies, and aftertreatment. Some of these technologies could reduce the efficiency compared to present day engines, and could require the use of low-sulfur fuel, which is more expensive to produce, adding to both future operating and capital costs.
Gaseous fuels such as natural gas, pure methane, ethane, liquefied petroleum gas, lighter flammable hydrocarbon derivatives, hydrogen, and blends of such fuels can be employed as substitutes for diesel fuel by modifying conventional diesel engines. Gaseous fuels are generally defined herein as fuels that are gaseous at atmospheric pressure and zero degrees Celsius. Whereas liquid fuels such as diesel are injected at very high pressures in order to atomize the fuel, gaseous fuels can be injected into an engine's combustion chamber at lower pressures because no extra energy is required for fuel atomization.
An advantage of substituting a gaseous fuel for diesel fuel is that a gaseous fuel can be selected that burns cleaner than diesel fuels, so that the present day desirable characteristics of diesel engines, namely high efficiency and high torque, can be preserved without requiring as much engine emission reduction technology.
Another advantage of gaseous fuels is that, as a resource, such fuels are more widely distributed around the world and the amount of proven reserves of natural gas is much greater, compared to proven oil reserves. In many markets around the world, natural gas is less expensive compared to diesel fuel on an energy equivalent basis.
Gaseous fuels can also come from renewable sources such as vent gases from garbage dumps and sewage treatment plants. Hydrogen can be produced with electricity generated from renewable sources such as wind power and hydro-electric dams.
However, some modifications are required to a conventional diesel engine to allow gaseous fuels to be substituted for diesel fuel. In a diesel engine, the heat produced by the mechanical compression of the fuel and air mixture auto-ignites the liquid diesel fuel charge at or near the end of the piston's compression stroke. Under the same conditions, gaseous fuels such as natural gas will not reliably auto-ignite. Accordingly, without re-designing engines to provide the conditions necessary to auto-ignite a gaseous fuel, in order to burn a gaseous fuel in a conventional engine with the same compression ratio, some additional device is required to assist with ignition of the gaseous fuel, such as a hot surface provided by a glow plug, or a fuel injection valve for introducing a pilot fuel. The pilot fuel can be a small quantity of diesel fuel, whereby the auto-ignition of the pilot fuel triggers the ignition of the gaseous fuel.
One of the problems with direct injection of gaseous fuels into a combustion chamber of an internal combustion engine is that there can be some variation in the amount of fuel injected from one injection event to the next. This is because in a gaseous-fuelled engine there are number of variables that can influence the amount of fuel that is injected. Some of these variables are not a factor for conventional diesel-fuelled engines or if it is, it is a factor to a different degree. For example, the in-cylinder pressure may vary slightly from cycle to cycle, and because it is desirable to maintain gaseous fuel injection pressure lower than the conventional diesel fuel injection pressure, the variations in in-cylinder pressure can have a larger effect on fuel mass flow rate. Also because of the lower mass density of gaseous fuels, the valve needle for a gaseous fuel injection valve can have a larger end surface area than a diesel fuel injection valve, whereby in-cylinder pressure can have a larger effect on the movement of the needle of a gaseous fuel injection valve. Variations in fuel temperature can also affect the mass density of gaseous fuels, introducing a variable that can cause variations in the amount of fuel that is introduced from one injection event to the next. FIG. 1 is a graph of the standard deviation of the actual delivered quantity of fuel against the duration of an injection event for an injection valve for gaseous fuels. The duration of an injection event is also referred to herein as “pulsewidth”. This graph shows that variability in the amount of fuel injected generally increases as pulsewidth decreases, with variability rising sharply once pulsewidth drops below a certain point. The actual values for axes on this graph are not shown, nor are they important since the actual values may be different for different engine systems and valve designs. However, the effect is significant and believed to be universal. Variability in the amount of fuel that is injected can cause inefficiencies, and if severe, such variability can cause unstable combustion. Accordingly, for gaseous-fuelled engines there is a need for an operating method that reduces variability in the amount of fuel that is injected.
Another problem encountered by gaseous-fuelled engines is that because gaseous fuels have a lower mass density, if injection pressure is kept constant, it can require lengthening the pulsewidth under high load conditions to inject the desired amount of fuel. Under high load conditions, with gaseous fuel it can reduce engine efficiency if the pulsewidth is too long.
Furthermore, if an engine employs exhaust gas circulation (EGR), which is a known technique for reducing emissions of NOx, one of the effects of high EGR rates is that it slows the rate of combustion. A benefit of using gaseous fuels with EGR is that the combustion of gaseous fuels produces less particulate matter and this means that less particulate matter is returned to the combustion chamber with the recirculated exhaust gas, allowing higher EGR rates. For the purposes of this disclosure, “high” amounts of EGR are considered to by EGR rates higher than 15%, and with a gaseous-fuelled engine it is possible to employ even higher EGR rates in the range of 30% to 40% under high load conditions. Accordingly, there is a need to modify gaseous fuel injection strategy to compensate for the slower combustion rate caused by high EGR rates and the lower mass density of gaseous fuels, especially when the engine is operating at high load and high speed, when a longer fuel injection pulsewidth can be particularly problematic.
While it has been demonstrated that by injecting a gaseous fuel directly into the combustion chamber of an internal combustion engine, it is possible to at least match the power output, performance, and efficiency of a conventional diesel engine, there are a number of factors that make gaseous-fuelled engines different from conventional diesel-fuelled engines, and these differences require operating methods that are different from those developed for conventional diesel engines.